WALTHAM, MA, October 11, 2011 – ImmunoGen, Inc. (Nasdaq:
IMGN) announced today that it has entered into a collaboration
agreement with Novartis to discover and develop targeted anticancer
therapeutics using antibodies to several antigen targets to be
named by Novartis.

Under this agreement, Novartis will pay a $45 million upfront
fee to ImmunoGen for exclusive rights to use the Company’s
Targeted Antibody Payload (TAP) technology with antibodies to the
specified number of antigen targets. For each target that results
in an anticancer therapeutic, ImmunoGen is entitled to receive
milestone payments potentially totaling $200.5 million plus
royalties on product sales, if any. The Company also is entitled to
receive financial compensation for research and for any
manufacturing done on behalf of Novartis. Novartis is responsible
for the development, manufacturing, and marketing of any products
resulting from this agreement.

“ImmunoGen has a valuable technology, and we’re
committed to developing our own pipeline of anticancer
therapeutics,” commented Daniel Junius, President and CEO of
ImmunoGen. “Our partnerships provide funding support for our
internal product programs and further development of our
technology. We plan to continue to establish new collaborations on
a selective basis.”

About ImmunoGen’s Targeted Antibody Payload (TAP)
Technology

ImmunoGen developed its TAP technology to achieve more
effective, better tolerated anticancer drugs. A TAP compound
consists of a tumor-targeting manufactured antibody with one of
ImmunoGen’s proprietary, highly potent cancer-killing agents
attached as a payload. The antibody serves to deliver the payload
specifically to tumor cells, and the payload serves to kill these
cells.

ImmunoGen’s technology portfolio uniquely provides:

A selection of highly potent cancer-killing payload
agents;
A selection of stable linkers for attachment of these agents to
antibodies; and
Means of attaching the payload to the antibody that retain the
desired structural and functional properties of the antibody.
Seven TAP compounds are now in clinical testing through
ImmunoGen’s own product programs and those of its
partners.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer therapeutics using
the Company’s expertise in tumor biology, monoclonal
antibodies and potent cancer-cell killing agents. The
Company’s TAP technology uses monoclonal antibodies to
deliver one of ImmunoGen’s proprietary cancer-cell killing
agents specifically to tumor cells. There are currently seven TAP
compounds in the clinic, with a wealth of clinical data reported
with the technology. ImmunoGen collaborative partners include
Amgen, Bayer Schering Pharma, Biogen Idec, Biotest, Genentech (a
member of the Roche Group), Novartis, and sanofi-aventis. The most
advanced compound using ImmunoGen’s TAP technology, T-DM1, is
in Phase III testing through the Company’s collaboration with
Genentech. More information about ImmunoGen can be found at
www.immunogen.com.

This press release includes forward-looking statements related
to the Company’s collaboration with Novartis. For these
statements, ImmunoGen claims the protection of the safe harbor for
forward-looking statements provided by the Private Securities
Litigation Reform Act of 1995. Various factors could cause
ImmunoGen’s actual results to differ materially from those
discussed or implied in the forward-looking statements, and you are
cautioned not to place undue reliance on these forward-looking
statements, which are current only as of the date of this release.
Factors that could cause future results to differ materially from
such expectations include, but are not limited to the timing and
outcome of ImmunoGen’s and the Company’s collaboration
partners’ research and clinical development processes; the
difficulties inherent in the development of novel pharmaceuticals,
including uncertainties as to the timing, expense and results of
preclinical studies, clinical trials and regulatory processes;
ImmunoGen’s ability to financially support its product
programs; ImmunoGen’s dependence on collaborative partners;
industry merger and acquisition activity; and other factors more
fully described in ImmunoGen’s Annual Report on Form 10-K for
the fiscal year ended June 30, 2010 and other reports filed with
the Securities and Exchange Commission.